Cleanliness of fluids is commonly measured using laser particle counters. Particle count can be represented in two different formats:
(i) ISO 4406 (1998) which measures total particle numbers per milliliter of fluid, by scaling the particle numbers in a range from 1-28, and the total particle count from 0.01 to 250,000. Particle count is measured in a 3-digit code X representing total particles >4 μm, Y representing total particles >6 μm, and Z representing total particles >14 μm.
(ii) NAS 1638 separates total particle count into 5 size ranges; 5-15 μm, 15-25 μm, 25-50 μm, 50-100 μm, and >100 μm.
Overall fluid particle code is a singular number that defines the worst particle count of the 5 scales. Total particle count is measured per 100 milliliters of fluid.
Laser particle counters can be limited in their ability to accurately measure small particles. Their ability to measure particles is often accepted as greater than 4.6 μm in size, leaving all particulate smaller than 4.6 μm as not reportable by this method.
It is also understood that the total number of particles in a typical in-service hydraulic or lubricating fluid contains 95% of contaminants that are less than 4.6 μm.
Existing configurations and design of electrostatic oil purifiers is varied. Electrostatic purifiers can often be categorized into 2 types:
A) Electrostatic purifiers induce a charge on the insoluble particles in the fluid and allow the capture of those particles in a collection medium. These electrostatic purifiers may be referred to as electrostatic oil cleaners.
B) Electrostatic purifiers that induce agglomeration, and make no attempt to contain flocculated particles within the fluid processing unit. These electrostatic purifiers depend on external mechanical filtration equipment to collect flocculated particles, and may be described as uncontained agglomeration purifiers.
Both purifier types depend upon a non-conductive environment to operate. Their performance can be impacted when conductive fluids, such as water, is introduced.
Voltage levels for type A purifiers are in the general range of about 10 kV to 15 kV. With reference to FIG. 1, reproduced from a paper presented by Akira Sasaki and Shinji Uchiyame, reference NCFP 102-1.2/SAE OH 2002-01-1352 (the Sasaki paper), it shows that the if oil flow is not precisely controlled, a net negative charge will be applied to the oil, as it continues on its path throughout the circulating oil system. Agglomeration of particles within the negatively charged oil occurs throughout the oil system. It can therefore be important to optimize a purifier design that minimizes voltage applied to the electrodes, so that external agglomeration does not occur.
Type B purifiers that initiate uncontained agglomeration may be represented by Munson WO 03/000406 A1, whereby a dual polarity electrode system is utilized to apply an appropriate voltage greater than 13 kV to apply a charge on particulate contaminant, and to a lesser degree on the fluid depending on its dielectric properties. The charge applied to the fluid and contaminants is dispersed throughout the entire fluid volume, whereby agglomeration occurs throughout the fluid system. Rate of agglomeration in a typical large gas turbine application will be dependant on parameters such as oil velocity and temperature in small hydraulic lines, electrostatic relaxation properties, fluid turbidity in large oil return lines to the turbine oil reservoir and air release properties of the fluid, along with oil residence time in the operating reservoir. Removal of particles is dependant on conventional mechanical filtration medias located external to the electrostatic high voltage electrodes and vessels containing same thereof.
Fouling of last-chance filters located in sensitive servo-valve hydraulic systems used to control gas turbine air-flow variable geometry is often initiated by the agglomeration process. This process is clearly undesirable.
There are several different concepts of cylindrical collectors that attempt to capture contaminants within the fluid processing unit. The Kawasaki U.S. Pat. No. 5,501,783 construction includes of a series of cylindrical components, including, multiple layers of pleated paper, interlined with cylindrical sheets of phenolic paper, and cylindrical electrodes. Each component is manufactured individually, cut to precise length, glued into a cylindrical shape, and finally installed into the outermost aluminum ground electrode. The integrity for eliminating oil channeling voids within the collector, when oil flows from bottom to top is dependent on the tight, and close tolerance between each successive layer of material. This type of construction is time consuming and leads to inconsistencies in product quality control and overall efficiencies of operation. Since oil flow is from bottom-to-top of collector in the '783 patent, there is opportunity for oil channeling along the walls of the phenolic paper inserts, see FIG. 2. The relatively large area within the paper pleats decreases the migration vector of small particles that may be centered between the pleats. The pleats form a triangle having approximately a 12 mm base and a 12 mm height.